The invention relates to an integrated circuit, comprising
an output stage which is organized as a series of at least two output transistors whose main current channels are connected parallel to one another between a supply terminal and an output of the output stage, PA1 a control circuit which is arranged to start, in response to a variation in an input signal on an input, a charging of respective control electrodes of the output transistors in a mutually delayed fashion in the order of the series.
An integrated circuit of this kind is known from U.S. Pat. No. 4,789,796.
During use of such an integrated circuit, the output is usually capacitively loaded. When the output voltage of the circuit changes from high to low, therefore, a capacitance must be discharged. Discharging is realized by way of a current through the output via the main current channels of the output transistors to the supply leads of the circuit. The supply leads act as an inductance, so that a variation in the current causes an inductive voltage across the supply leads which is proportional to the time derivative of the current.
Due to the inductive voltage across the supply leads, the power supply terminals, such as ground outside the envelope of the integrated circuit and ground within the integrated circuit, no longer carry the same potential. This disturbs the operation of the circuit.
This disturbance is reduced by the known circuit, in that the peak value of the time derivative of the current is reduced, subject to the secondary condition that the capacitance must have been discharged within a specified period of time. Mutually delayed starting of the charging of the control electrodes ensures that the control electrodes of the output transistors do not reach their maximum voltage level simultaneously. Therefore, the instant at which the increase of the current through the various output transistors is maximum differs. As a result, the peak value of the time derivative of the current through the output is smaller than if the current in the at least two output transistors were to increase simultaneously.
As the number of output transistors is larger, in principle an increasingly lower peak value can thus be realized in the derivative of the current. However, this requires a complex circuit in which notably the wiring occupies a large surface area on the integrated circuit.
U.S. Pat. No. 4,783,601 discloses another output stage. This output stage consists of one output transistor whose main current channel is connected between a first supply terminal and the output.
To this end, the output stage comprises a control circuit which is arranged for charging the control electrode of the output transistor in response to a variation in an input signal on the input. The control circuit comprises several charging current branches, all but one of which are switched off when the voltage on the control electrode exceeds a threshold value. Thus, at the beginning of charging the charging current of the control electrode is larger than that at the end of charging.
By making the charging current larger at the beginning, the time required for reaching the maximum current through the output is reduced, without increasing the peak value of the derivative of the current. The charging time required can be further reduced by connecting more charging current branches in parallel and by switching off these branches at different threshold values.
The peak value of the time derivative of the current of the output stage published in U.S. Pat. No. 4,783,601, however, is very sensitive to a spread in the parameters of the integrated circuit. Moreover, the output stage does not operate very well in the case of low supply voltages, because it is difficult to realize sufficiently accurate adjustment of the threshold value of the voltage on the control electrode at which the various charging current branches must be switched off. Furthermore, this circuit cannot be optimally used for different supply voltages, because the switching off of the branches is dependent on the supply voltage.